Hydrogen sulfide (British English: hydrogen sulphide) is an extremely dangerous chemical compound with the formulaH2S. It is a colorless, very poisonous, flammable gas with the characteristic foul odor of rotten eggs. It must be emphasised that the unpleasant odour of H2S is detectable at concentrations of 0.02 ppm, but may not be detectable at concentrations above 100 ppm due to rapid loss of the sense of smell, which is the level classified as "Immediately Dangerous to Life and Health".

The major hazards of H2S are its ability to cause rapid damage to health or sudden death due to accidental exposure and metal integrity failure due to general corrosion or cracking. There are maximum acceptable H2S concentration limits in sale quality oil and gas, which require any excess H2S to be removed by H2S scavenging process.

Properties

Hydrogen sulfide is slightly heavier than air; a mixture of H2S and air is explosive. Hydrogen sulfide and oxygen burn with a blue flame to form sulfur dioxide (SO2) and water. In general, hydrogen sulfide acts as a reducing agent.

At high temperature or in the presence of catalysts, sulfur dioxide can be made to react with hydrogen sulfide to form elemental sulfur and water. This is exploited in the Claus process, the main way to convert hydrogen sulfide into elemental sulfur.

Hydrogen sulfide is slightly soluble in water and acts as a weak acid, giving the hydrosulfide ion HS− (pKa = 6.9 in 0.01-0.1 mol/litre solutions at 18 °C) and the sulfide ion S2− (pKa = 11.96). A solution of hydrogen sulfide in water, known as sulfhydric acid or hydrosulfuric acid,[5] is initially clear but over time turns cloudy. This is due to the slow reaction of hydrogen sulfide with the oxygen dissolved in water, yielding elemental sulfur, which precipitates out.

Hydrogen sulfide reacts with metal ions to form metal sulfides, which may be considered the salts of hydrogen sulfide. Some ores are sulfides. Metal sulfides often have a dark color. Lead(II) acetate paper is used to detect hydrogen sulfide because it turns grey in the presence of the gas as lead(II) sulfide is produced. Reacting metal sulfides with strong acid liberates hydrogen sulfide.

If gaseous hydrogen sulfide is put into contact with concentrated nitric acid, it explodes.

Personal safety

Hydrogen sulfide is a highly toxic and flammable gas (flammable range: 4.3–46%). Being heavier than air, it tends to accumulate at the bottom of poorly ventilated spaces. Although very pungent at first, it quickly deadens the sense of smell when above 100ppm, so potential victims may be unaware of its presence until it is too late. For safe handling procedures, a hydrogen sulfide material safety data sheet (MSDS) should be consulted.[6]

Since hydrogen sulfide occurs naturally in the body, the environment and the gut, enzymes exist in the body capable of detoxifying it by oxidation to (harmless) sulfate.[8] Hence, low levels of hydrogen sulfide may be tolerated indefinitely.

At some threshold level, believed to average around 300–350 ppm, the oxidative enzymes become overwhelmed. Many personal safety gas detectors, such as those used by utility, sewage and petrochemical workers, are set to alarm at as low as 5 to 10 ppm and to go into high alarm at 15 ppm.

Symptoms

Exposure to lower concentrations can result in eye irritation, a sore throat and cough, nausea, shortness of breath, and fluid in the lungs. These effects are believed to be due to the fact that hydrogen sulfide combines with alkali present in moist surface tissues to form sodium sulfide, a caustic.[12] These symptoms usually go away in a few weeks.

Long-term, low-level exposure may result in fatigue, loss of appetite, headaches, irritability, poor memory, and dizziness. Chronic exposure to low level H2S (around 2 ppm) has been implicated in increased miscarriage and reproductive health issues among Russian and Finnish wood pulp workers,[13] but the reports have not (as of circa 1995) been replicated.

0.0047 ppm is the recognition threshold, the concentration at which 50% of humans can detect the characteristic odor of hydrogen sulfide,[14] normally described as resembling "a rotten egg".

Less than 10 ppm has an exposure limit of 8 hours per day.

10–20 ppm is the borderline concentration for eye irritation.

50–100 ppm leads to eye damage.

At 100–150 ppm the olfactory nerve is paralyzed after a few inhalations, and the sense of smell disappears, often together with awareness of danger.[15][16]

530–1000 ppm causes strong stimulation of the central nervous system and rapid breathing, leading to loss of breathing.

800 ppm is the lethal concentration for 50% of humans for 5 minutes exposure (LC50).

Concentrations over 1000 ppm cause immediate collapse with loss of breathing, even after inhalation of a single breath.

Although respiratory paralysis may be immediate, it can also be delayed up to 72 hours.[17]

Hydrogen sulfide was used by the British Army as a chemical agent during World War I. It was not considered to be an ideal war gas, but, while other gases were in short supply, it was used on two occasions in 1916.[18]

A dump of toxic waste containing hydrogen sulfide is believed to have caused 17 deaths and thousands of illnesses in Abidjan, on the West Africa coast, in the 2006 Côte d'Ivoire toxic waste dump.

Asset integrity

H2S induced sulfide stress cracking failure.

H2S can and does cause metal loss attack which generally tends to be localized due to the tendency for iron sulfide films to be formed on a steel surface (pH dependent) which breakdown or spall locally leading to pitting. However, the primary concern with the presence of H2S is the risk of causing sulfide stress cracking (SSC). This has long been recognized and prompted the development of the NACE Standard MR0175 "Sulfide Stress Cracking Resistant Metallic Materials for Oilfield Equipment".

The cracking caused by H2S can result in catastrophic failure in certain circumstances. It it is generally of greater concern to downhole and topside equipment. It is often also difficult to detect early and monitor in practice, and to subsequently control. Thus emphasis is firmly placed on identifying the risk at the materials selection stage and hence to select a material which is not susceptible to cracking rather than try to control the situation by use of a corrosion inhibitor.

Cracking mechanism

Combinations of tensile stresses and specific corrosion environments are one of the most important causes of catastrophic cracking.Generation and diffusion of atomic hydrogen into the metal matrix.

Susceptible alloys, especially steels, react with hydrogen sulfide, forming metal sulfides and atomic hydrogen as corrosion byproducts. The atomic hydrogen under the concentration gradient diffuses into the metal matrix where it can get trapped at inclusions and grain boundaries as well as can pass right through the metal. Higher strength (> ca. 70 ksi yield strength) and hardness steels are most susceptible due to localised hydrogen embrittlement caused by the trapped hydrogen atoms which, once a crack has initiated, will concentrate just ahead of the crack tip and promote its propagation through the metal.

Crack initiation is not dependent on pit formation but can occur at any surface stress raiser or discontinuity in the presence of an applied stress. As with all localized corrosion processes, there is an induction period before a crack initiates which will depend on the stress level, (local) hardness, the material composition/microstructure and hydrogen permeation rate. At low stress levels cracks will tend to be inter-granular whereas at high stress levels they can be trans-granular.

H2S Removal

Hydrogen sulfide is commonly found in natural gas, biogas, and LPG. It can be removed in a number of ways, depending on the carrying fluids.

Hydrodesulfurization

Production

Hydrogen sulfide is most commonly obtained by its separation from sour gas, which is natural gas with high content of H2S. It can also be produced by reacting hydrogen gas with molten elemental sulfur at about 450 °C. Hydrocarbons can replace hydrogen in this process.[20]

Analytical chemistry

For well over a century, hydrogen sulfide was important in analytical chemistry, in the qualitative inorganic analysis of metal ions. In these analyses, heavy metal (and nonmetal) ions (e.g., Pb(II), Cu(II), Hg(II), As(III)) are precipitated from solution upon exposure to H2S. The components of the resulting precipitate redissolve with some selectivity.

For small-scale laboratory use in analytic chemistry, the use of thioacetamide has superseded H2S as a source of sulfide ions.

Precursor to metal sulfides

As indicated above, many metal ions react with hydrogen sulfide to give the corresponding metal sulfides. This conversion is widely exploited. For example, gases or waters contaminated by hydrogen sulfide can be cleaned with metal sulfides. In the purification of metal ores by flotation, mineral powders are often treated with hydrogen sulfide to enhance the separation. Metal parts are sometimes passivated with hydrogen sulfide. Catalysts used in hydrodesulfurization are routinely activated with hydrogen sulfide, and the behavior of metallic catalysts used in other parts of a refinery is also modified using hydrogen sulfide.

Miscellaneous applications

Suicides

The gas, produced by mixing certain household ingredients, was used in a suicide wave in 2008 in Japan.[21] The wave prompted staff at Tokyo's suicide prevention center to set up a special hot line during "Golden Week", as they received an increase in calls from people wanting to kill themselves during the annual May holiday.[22]

As of 2010, this phenomenon has occurred in a number of US cities (and in Putney West London, England), prompting warnings to those arriving at the site of the suicide.[23] These first responders, such as emergency services workers or family members are at risk of death from inhaling lethal quantities of the gas, or by fire.[24][25] Local governments have also initiated campaigns to prevent such suicides.

Function in the body

Hydrogen sulfide is produced in small amounts by some cells of the mammalian body and has a number of biological signaling functions. (Only two other such gases are currently known: nitric oxide (NO) and carbon monoxide (CO).)

Though both nitric oxide and hydrogen sulfide have been shown to relax blood vessels, their mechanisms of action are different: while NO activates the enzyme guanylyl cyclase, H2S activates ATP-sensitive potassium channel in smooth muscle cells. Researchers are not clear how the vessel-relaxing responsibilities are shared between nitric oxide and hydrogen sulfide. However there exists some evidence to suggest that nitric oxide does most of the vessel-relaxing work in large vessels and hydrogen sulfide is responsible for similar action in smaller blood vessels.[30]

Like nitric oxide, hydrogen sulfide is involved in the relaxation of smooth muscle that causes erection of the penis, presenting possible new therapy opportunities for erectile dysfunction.[31][32]

In Alzheimer's disease the brain's hydrogen sulfide concentration is severely decreased.[33] In trisomy 21 (the most common form of Down syndrome) the body produces an excess of hydrogen sulfide.[28] Hydrogen sulfide is also involved in the disease process of type 1 diabetes. The beta cells of the pancreas in type 1 diabetes produce an excess of the gas, leading to the death of beta cells and to a reduced production of insulin by those that remain.[30]

Induced hypothermia/suspended animation

In 2005, it was shown that mice can be put into a state of suspended animation-like hypothermia by applying a low dosage of hydrogen sulfide (81 ppmH2S) in the air. The breathing rate of the animals sank from 120 to 10 breaths per minute and their temperature fell from 37 °C to just 2 °C above ambient temperature (in effect, they had become cold-blooded). The mice survived this procedure for 6 hours and afterwards showed no negative health consequences.[34] In 2006 it was shown that the blood pressure of mice treated in this fashion with hydrogen sulfide did not significantly decrease.[35]

A similar process known as hibernation occurs naturally in many mammals and also in toads, but not in mice. (Mice can fall into a state called clinical torpor when food shortage occurs). If the H2S-induced hibernation can be made to work in humans, it could be useful in the emergency management of severely injured patients, and in the conservation of donated organs. In 2008, hypothermia induced by hydrogen sulfide for 48 hours was shown to reduce the extent of brain damage caused by experimental stroke in rats.[36]

As mentioned above, hydrogen sulfide binds to cytochrome oxidase and thereby prevents oxygen from binding, which leads to the dramatic slowdown of metabolism. Animals and humans naturally produce some hydrogen sulfide in their body; researchers have proposed that the gas is used to regulate metabolic activity and body temperature, which would explain the above findings.[37]

Two recent studies cast doubt that the effect can be achieved in larger mammals. A 2008 study failed to reproduce the effect in pigs, concluding that the effects seen in mice were not present in larger mammals.[38] Likewise a paper by Haouzi et al. noted that there is no induction of hypometabolism in sheep, either.[39]

At the February 2010 TED conference, Mark Roth announced that hydrogen sulfide induced hypothermia had completed Phase I clinical trials.[40] The clinical trials commissioned by the company he helped found, Ikaria, were however withdrawn or terminated by August 2011.[41][42]

As organic matter decays under low-oxygen (or hypoxic) conditions (such as in swamps, eutrophic lakes or dead zones of oceans), sulfate-reducing bacteria will use the sulfates present in the water to oxidize the organic matter, producing hydrogen sulfide as waste. Some of the hydrogen sulfide will react with metal ions in the water to produce metal sulfides, which are not water soluble. These metal sulfides, such as ferrous sulfide FeS, are often black or brown, leading to the dark color of sludge.

Several groups of bacteria can use hydrogen sulfide as fuel, oxidizing it to elemental sulfur or to sulfate by using dissolved oxygen, metal oxides (e.g., Fe oxyhydroxides and Mn oxides) or nitrate as oxidant.[43]

Organic residues from these extinction boundaries indicate that the oceans were anoxic (oxygen-depleted) and had species of shallow plankton that metabolized H2S. The formation of H2S may have been initiated by massive volcanic eruptions, which emitted carbon dioxide and methane into the atmosphere, which warmed the oceans, lowering their capacity to absorb oxygen that would otherwise oxidize H2S. The increased levels of hydrogen sulfide could have killed oxygen-generating plants as well as depleted the ozone layer, causing further stress. Small H2S blooms have been detected in modern times in the Dead Sea and in the Atlantic ocean off the coast of Namibia.[44]